For Each GroupBoxCntrol As Control In Me.Controls
            If TypeOf GroupBoxCntrol Is GroupBox Then
                For Each controls As Control In GroupBoxCntrol.Controls
                    If TypeOf (controls) Is System.Windows.Forms.TextBox Then
                        abrakadabra(controls)
                    End If
                Next
            End If
        Next

public static IEnumerable<Control> GetControls(this Control parent, Func<Control, bool> predicate=null)
        {
            return
                parent.Controls.OfType<Control>()
                    .SelectMany(c => GetControls(c))
                    .Concat(new[] { parent })
                    .Where(c => predicate == null || predicate(c));
        }